A recent trend is toward the laying of more and more underground pipelines for the transportation of petroleum and petroleum products in locations where they are handled. For such underground installations a Japanese Ministerial ordinance "Oil products pipeline business law" enacted in 1972 makes it an obligation to provide a leak detector which would promptly detect any leak from any piping.
Prior art detection methods include:
(1) Flow difference system PA1 (2) Pressure grade pilot system PA1 (3) Differential pressure pilot system PA1 (4) Acoustic sound detection system PA1 (5) Gas or perfume detection system PA1 (6) Special underground cable system
These known systems have a number of drawbacks. The systems (1) and (2) fail to detect minor leaks exactly, systems (3) and (4) are not adapted for continuous monitoring, and system (5) demands an unduly large number of detection instruments when applied to long-distance pipelines. The last-mentioned system (6), disclosed in my copending patent applications (Japanese Pat. Appln. Nos. 19898/73 & 57960/73), comprises laying a special cable composed of two electric conductors covered by an oil-soluble insulation in parallel with a pipeline so that, in case of an oil leak, the leaking fluid passes into the cable and dissolves the insulation to lower the electric resistance between the conductors and thereby make the leak known. This system functions rather instably, however, because the leaking oil itself is an insulating material and its presence can in some cases prevent the drop of inter-conductor resistance or short-circuiting between the conductors despite the dissolution of the oil-soluble insulation. In addition, the use of a more readily soluble insulation as the wrapping for the conductors can shorten the cable life to an impractical extent. Conversely, if a less soluble insulation is employed for added durability, it will take so much time for dissolution as to make quick detection difficult.